Microelectronic devices are used in cell phones, pagers, personal digital assistants, computers, and many other products. A packaged microelectronic device can include a microelectronic die, an interposer substrate or lead frame attached to the die, and a molded casing around the die. One process for packaging microelectronic devices at the die level includes (a) attaching individual dies to an interposer substrate, (b) wire-bonding contacts on the dies to corresponding components on the interposer substrate, and (c) encapsulating the dies with a molding compound.
FIG. 1 is a top cutaway isometric view of an existing microelectronic device 10. The microelectronic device 10 includes a substrate 6 and a microelectronic die 4 attached to the substrate 6. The microelectronic device 10 shown in FIG. 1 illustrates the substrate 6 and the die 4 before encapsulating the die 4 with an encapsulation compound. The substrate 6 includes a first array of ball-pads 2, a second array of terminal pads 3 proximate to a slot 18, and a trace 22 or other type of conductive line between each ball-pad 2 and corresponding terminal pad 3. The slot 18 extends lengthwise along a medial portion of the substrate 6. The substrate 6 is an interposing device that provides an array of ball-pads for coupling very small contacts on the microelectronic die to another type of device.
The microelectronic die 4 can include a plurality of small contacts 5 and an integrated circuit 7 (shown schematically) coupled to the contacts 5. The contacts are arranged in an array on the microelectronic die 4 so that the contacts 5 are aligned with or otherwise accessible through the slot 18 in the substrate 6. A plurality of wire-bonds 9 electrically couple the contacts 5 of the die 4 to corresponding terminal pads 3 on the substrate 6. A wire bonder forms the wire-bonds 9 between the die 4 and the substrate 6 with a capillary, which feeds wire through a central aperture. For example, a molten ball formed at a protruding end of the wire and the capillary are pressed against one of the contacts 5 to attach the end of the wire to the die 4. The capillary then moves upward and laterally to attach the wire to the terminal pad 3 on the substrate 6.
In other applications, the capillary may attach only one end of the wire-bond. For example, a microelectronic device can include a die and a wire-bond having a first end attached to the die and a second, free end projecting away from the die. In these applications, the wire bonder attaches the end of the wire to the die and then severs the wire. The wire is typically severed by placing an electrical potential on the wire and an opposite potential on a single electrode adjacent to the wire to generate a spark between the electrode and the wire. The single electrode is positioned at a desired point to sever the wire so that the spark cuts the wire and forms a ball at the end of the wire. As set forth in U.S. Pat. No. 5,773,780, the wire bonders can also include a light source that directs light toward a segment of the wire to stabilize the spark. These approaches, however, have several drawbacks. For example, the point at which the wire is severed is not consistent and predictable because the spark can occur between the electrode and anywhere along a segment of the wire proximate to the electrode. Thus, the length of the wire-bond may be shorter or longer than desired. Moreover, it is difficult and expensive to retrofit existing wiring bonding machines with the light source. Accordingly, there exists a need to improve the process of severing the wire during wire-bonding.